1.Field of the Invention
The present invention relates to a multistage vacuum pump. It also extends to a pumping installation including such a pump.
2.Description of the Related Art
A vacuum pump is a device capable of extracting and exhausting molecules of gas so as to lower the pressure inside a chamber. A “rough” vacuum is defined as pressures greater than about 1 millibar (mbar). Under such circumstances, a so-called “primary” pump is used, such as a vane pump. A “medium” vacuum corresponds to pressures lying in the range 1 mbar to 10−3 mbar. A “high” vacuum relates to pressures less than about 10−3 mbar and down to about 10−7 mbar, beyond which the vacuum is said to be “ultrahigh”. If it is desired to obtain a high vacuum in an installation, a primary pump has a secondary pump coupled thereto, e.g. a turbomolecular pump.
When a plurality of pumping stages of the same type are coupled together in series in the same pump, the pump is said to be multistage pump. For example, a two-stage vacuum pump that is in widespread use has two stages. The first stage in the flow direction, or suction stage (commonly referred to as the “low pressure” stage since this is the stage in which the lowest pressure is generated) is of large size so as to maximize pumping flow rate. The second stage in the flow direction, or delivery stage (commonly called the “high pressure” stage since this is the stage that operates at the highest pressure) is generally of smaller size, with a lower flow rate.
The invention relates in particular to an installation having at least two chambers requiring different vacuum levels and each connected to a pump unit. The presence of a plurality of pumps having different characteristics in such an installation turns out to be particularly costly in terms of investment, maintenance, and energy consumption. There is thus a need for an installation requiring a small number of pumps for equivalent performance and at a cost that is lower than that of presently known installations.
By way of example, document U.S. Pat. No. 5,733,104 describes an installation comprising four chambers in succession in which a vacuum is required. The last three chambers are connected to a secondary pumping unit comprising two turbomolecular stages and a molecular stage. The two turbomolecular stages have their admission inlets respectively connected to the last two chambers of the installation, the second stage also being connected to the outlet of the first stage. The molecular stage is connected to the outlet of the second turbomolecular stage and possesses an admission inlet connected to the third chamber. These stages have their rotors driven by a common transmission shaft, itself driven by a single motor and a single control member. The first chamber is connected to a dry primary pump such as a diaphragm pump. The dry primary pump is installed in series with the secondary pumping unit and is connected via its suction inlet to the outlet of the molecular stage. A duct for pumping the first chamber is connected to the same inlet.
That solution suffers from the drawback that only one flow is possible compatible with the limited flow rate of the diaphragm pump. In addition, it can be operated only with stable pressure ratios from one chamber to another, even if the compression ratio of the molecular portion and the presence of the cold trap for compressible vapors make it possible to interrupt the operation of the diaphragm pump in order to extend the lifetime of the diaphragm.
Document U.S. Pat. No. 3,668,393 shows an installation, e.g. an electron microscope, comprising firstly an enclosure in which a high level of vacuum is required, and secondly additional compartments that are evacuated. The installation also has a pumping group comprising a turbomolecular arrangement and a two-stage primary pump. The turbomolecular arrangement comprises a main stage connected to the enclosure and an auxiliary stage connected to the additional compartment. Both stages are contained in a common housing and are connected via a common transmission shaft to a single motor; nevertheless a relatively leaktight partition separates the outlets from the two pumps which do not communicate with each other, each being delivered to a different stage of the associated pump. The primary pump comprises a high pressure stage whose admission inlet is connected to the outlet of the main pump of the turbomolecular arrangement and whose delivery outlet continuously delivers gas to the low pressure stage of the primary pump. The low pressure stage is then connected to the high pressure stage via a transfer channel to which the outlet of the auxiliary pump is connected via a valve. The flow from the low pressure stage of the primary pump is mixed with the flow coming from the auxiliary turbomolecular pump at the high pressure stage thereof so that the gases pumped via those two paths can subsequently be exhausted to the outside.
Such an installation makes it possible simultaneously to have two different levels of vacuum in two distinct spaces while using a single conventional two-stage primary pump and an arrangement further including two secondary pumps.
That solution suffers from the drawback of there being a high degree of interdependence between the vacuum levels and the flows obtained through each orifice of the vane pump. The system thus operates only with small pressure variations in the auxiliary pumping system, thus limiting its use to well-defined experimental conditions.